Has America's love for cars gone downhill?

A series of reports and studies are painting an intriguing picture of the future of transportation in the U.S. -- one that involves a possible shift away from cars.

The American Public Transportation Association (APTA) said in March that Americans took 10.7 billion trips on public transportation last year, the highest annual figure for public transit ridership in 57 years. And while the number of vehicle miles traveled on the nation's roads was up 0.3 percent in 2013, public transportation use during the same time period increased by 1.1 percent.

"There is a fundamental shift going on in the way we move about our communities," APTA President and CEO Michael Melaniphy said in a statement. "People in record numbers are demanding more public transit services, and communities are benefiting with strong economic growth."

And just this week, the University of Michigan's Transportation Research Institute noted that, despite continued growth of the U.S. population, fuel consumption by American drivers in light-duty vehicles -- cars, pickup trucks, SUVs and vans -- is lower now than it was 15 years ago.

"The [fuel consumption] decline of 11 percent since 2004 reflects the decline in distance driven and the improvement in vehicle fuel economy," Michael Sivak, a research professor at the Institute and the study's author, said on the university's website.

When Sivak broke down fuel-consumption rates by household, per person, per licensed driver and by registered vehicle, he found all four sets of data were 13 percent to 18 percent lower in 2012 than compared to their peak years in 2003 and 2004.

Economic factors are certain to have contributed to that data, as higher fuel prices, decreased business and tightened personal finances during the recession meant less travel, increased telecommuting and a rising demand for public transportation. The automotive industry, meanwhile, has come roaring back from its recession lows, as the economy recovers and consumers unleash all that pent-up demand for a new vehicle.

But researchers believe that, while America's long love affair with the car isn't over, it may be starting to cool.

"I do think that the age of rapid automobilization, with its exponential growth that we saw for decades, that period is over," said Colleen Callahan, deputy director of UCLA's Luskin Center for Innovation. "So those rapid increases, from decade to decade -- that time has really ended, and... we now are in a time where any increase might be more modest."

She also agrees that, while economic factors are still a driving force in consumer demand for cars, there does appear to be a generational shift taking place. Callahan points to a 2012 study by her colleagues at UCLA's Institute of Transportation Studies that found that younger Americans are making fewer trips by car and "traveling considerably fewer miles" than teens and young adults in previous decades.

Certain factors appear to be contributing to that change, including higher unemployment rates and fuel prices, as well as the growing trend toward urbanization.

However, "speaking from the perspective of someone who lives in what many people call the car culture center of U.S., I have seen in the past decade a growing energy and momentum around non-automobile transportation," Callahan said.

Given America's size, demographics and infrastructure, there is likely to always be a need -- even a dependency -- on cars and other vehicles in some regions.

"But the trajectory that we're on, up until about a decade ago, it was really unsustainable," noted Callahan. "There was steep growth over time, in terms of automobile ownership and miles driven. At some point it's got to end... this exponential growth cannot continue into the future, and I think we're finally reaching that plateau."